For example, in in-vehicle networks mounted on automobiles, a number of terminals, such as electronic control units (ECU), are connected through a network bus of a standard, such as controller area network (CAN) and FlexRay (registered trademark). Each of the terminals, such as ECUs, holds firmware inside, and operates by executing the processing described in the firmware. The firmware stored in the terminal is updated, to expand the function of the terminal, and take countermeasures against newly found vulnerability. In the case of updating software, such as the firmware in the terminal, on a network on which the security measure is not entirely perfect, as in in-vehicle networks, it is desired to guarantee proper update of software.